The present invention relates to gas turbine engines, and more particularly, to an ultra low NOx emissions combustion system for gas turbine engines.
Low NOx emissions from a gas turbine engine, of below 10 volume parts per million (ppmv), are becoming important criteria in the selection of gas turbine engines for power plant applications. Some installations in non-attainment area in the United States are demanding even lower NOx emissions of less than 5 ppmv. The challenging NOx emission requirements must be achieved without compromising the more conventional constraints on gas turbine engines, of durability, low operating costs and high efficiency.
The main factor governing nitrogen oxide formation is temperature. One of the most attractive methods of reducing flame temperatures involves using Lean Premixed combustion, in which reductions in flame temperatures are readily accomplished by increasing the air content in a given fuel/air mixture. This method is often referred to as a Dry-Low-Emissions (DLE) to distinguish it from Wet NOx control by water or steam injection, and highlight the low emissions in which NOx levels down to 10 ppmv can be achieved.
However, flame stability decreases rapidly under these lean combustion conditions and the combustor may be operating close to its blow-out limit. In addition, severe constraints are imposed on the homogeneity of the fuel/air mixture since leaner than average pockets of mixture may lead to stability problems and richer than average pockets will lead to unacceptably high NOx emissions. The emission of carbon monoxide as a tracer for combustion efficiency will increase at leaner mixtures for a given combustor due to the exponential decrease in chemical reaction kinetics. Engine reliability and durability are of major concern under lean combustion conditions due to high-pressure fluctuations enforced by flame instabilities in the combustor.
It is well known in the industry that catalytic combustion can be used as an ultra-lean premixed combustion process where a catalyst is used to initiate and promote chemical reactions in a premixed fuel/air mixture beyond flammability limits that would otherwise not burn. This permits a reduction of peak combustion temperatures to levels below 1,650 K, and NOx emissions less than 5 ppmv can be achieved.
Nevertheless, major challenges have prevented the implementation of catalytic combustors in a gas turbine engine. Catalyst operation and durability demand a very tight control over the engine and catalyst inlet operating parameters. As shown in FIG. 1, which is a graphical representation of a normalized catalyst operating window and the compressor discharge temperature variations from engine idle to full power, the compressor discharge temperatures increase from engine idle to full power over a range typically more than three times that which, as being defined between lines M and N, is acceptable for catalyst operation.
In the prior art, most Catalyst combustion systems utilize a pre-burner to increase compressor discharge air temperature at engine low power conditions where the compressor discharge air temperature is below catalyst ignition temperature. Other major problems in catalyst operation include ignition, engine start-up and catalyst warm-up which cannot be performed with the catalyst. A separate fuel system is required. Any liquid fuel combustion has to be introduced downstream of the catalyst to prevent liquid fuel flooding the catalyst in case of ignition failure. Because of the narrow range of acceptable catalyst inlet temperatures, the catalyst has to be designed for full power operating conditions. As the engine decelerates the fuel/air mass ratio decreases. Generally, this compromises the catalyst and engine performance under part load conditions, thereby resulting in emissions leading to very high NOx and CO levels. The catalyst durability is affected by engine transient operation since catalyst operation is a delicate balancing act between catalyst ignition (blow-out) and catalyst burn-out. In this sense, turn-down of the catalyst system becomes a serious operability and durability issue. In the case when the pre-burner is used for part load or the entire operating range of the engine, the pre-burner then becomes the main source of NOx emissions from the engine. In addition, hot streaks from the pre-burner are very likely to damage catalyst hardware directly or act as sources of auto-ignition within the fuel/air mixing duct upstream of the catalyst, and impose a substantial risk to catalyst and engine operation. A pre-burner also substantially increases the combustor pressure drop by an additional 1.5% to 2.5%, which directly affects engine specific fuel consumption.
Efforts have been made to improve catalytic combustors for gas turbine engines. One example of the improvements is described in U.S. Pat. No. 5,623,819, issued to Bowker et al. on Apr. 29, 1997. Bowker et al. describe a low NOx generating combustor in which a first lean mixture of fuel and air is pre-heated by transferring heat from hot gas discharging from the combustor. The pre-heated first fuel/air mixture is then catalyzed in a catalytic reactor and then combusted so as to produce a hot gas having a temperature in excess of the ignition temperature of the fuel. Second and third lean mixtures of fuel and air are then sequentially introduced into the hot gas, thereby raising their temperatures above the ignition temperature and causing homogeneous combustion of the second and third fuel/air mixtures. This homogeneous combustion is enhanced by the presence of the free radicals created during the catalyzing of the first fuel/air mixture. In addition, the catalytic reactor acts as a pilot that imparts stability to the combustion of the lean second and third fuel/air mixtures.
Another example of the improvements is described in U.S. Pat. No. 5,850,731, issued to Beebe et al. on Dec. 22, 1998. Beebe et al. describe a combustor for gas turbine engines and a method of operating the combustor under low, mid-range and high-load conditions. At the start-up or low-load levels, fuel and compressor discharge air are supplied to the diffusion flame combustion zone to provide combustion products for the turbine. At mid-range operating conditions, the products of combustion from the diffusion flame combustion zone are mixed with additional hydrocarbon fuel for combustion in the presence of a catalyst in the catalytic combustion zone. Because the fuel air mixture in the catalytic reactor bed is lean, the combustion reaction temperature is too low to produce thermal NOx. Under high-load conditions a lean direct injection of fuel/air is provided in a post-catalytic combustion zone where auto-ignition occurs with the reactions going to completion in the transition between the combustor and turbine sections. In the post-catalytic combustion zone, the combustion temperature is low and the residence time in the transition piece is short, hence minimizing thermal NOx.
Nevertheless, there is still a need for further improvements of low emissions combustors for gas turbine engines that will allow minimizing the emissions of the NOx, CO and unburned hydrocarbon (UHC) simultaneously, over the entire operating range of the gas turbine engine.
It is an object of the present invention to provide an ultra-low emissions combustion system for gas turbine engines which permits minimizing the emissions of NOx, CO and UHC simultaneously over the entire operating range of the gas turbine engine.
It is another object of the present invention to provide a combustor for a gas turbine engine and a method of operating the combustor which combines the advantages of a conventional Dry-low-emissions combustion system with a catalytic combustion system.
It is a further object of the present invention to provide a method for operating a combustor for a gas turbine engine having a conventional Dry-low-emissions combustion system and a Catalyst combustion system which can operate separately, to achieve low emissions of NOx, CO and UHC simultaneously over the entire operating range of the gas turbine engine.
In accordance with one aspect of the present invention, a method of operating a combustor for a gas turbine engine over an entire operating range thereof at high engine efficiency while minimizing emissions of nitrogen oxides NOx and carbon monoxide CO from the engine, comprises: under low-load conditions supplying a fuel and an air flow to a Dry-low-emissions (DLE) combustion system of the combustor to generate combustion products; under high-load conditions stopping the fuel and air flow to the DLE combustor system and supplying a fuel and air flow to a Catalyst (CAT) combustion system of the combustor to generate combustor products; and the low and high load conditions being defined by a predetermined power level, the predetermined power level being associated with an adequate catalyst inlet temperature so that the combustion procedure of the combustor switches over from the DLE combustor system to the CAT combustor system when the adequate catalyst inlet temperature can be achieved, resulting from increasing of an engine power level.
The catalyst inlet temperature is controlled within catalyst operating conditions for engine loads between the predetermined power level and the full-load condition, preferably by adjusting the air flow to the CAT combustor system and adding heat to the CAT combustor system from the combustor cooling heat transfer. It is preferable to maintain the combustion products from either one of the DLE and CAT combustor systems inside the combustor for an extended residence time in order to convert CO formed in the combustion products to CO2.
In accordance with another aspect of the present invention a low-emissions combustion system for a gas turbine engine is provided. The system comprises a Dry-low-emissions (DLE) combustion sub-system for generating combustion products under a lean premixed fuel/air condition, and a Catalyst (CAT) combustion sub-system for generating combustion products under a lean premixed fuel/air condition in the presence of a catalyst. The combustion system further includes a combustor scroll connected to the DLE and CAT combustion sub-systems for delivering the combustion products in adequate inlet conditions, to an annular turbine of the engine. A fuel injection sub-system for injecting fuel into the respective DLE and CAT combustion sub-systems is provided; and an air supply sub-system for supplying air to the respective DLE and CAT combustion sub-systems is also provided. The combustion system includes a control sub-system for controlling the fuel injection and air supply sub-systems to selectively inject fuel and selectively supply air to the respective DLE and CAT combustion sub-systems.
The combustor scroll preferably includes a transition section connecting the combustor scroll to the DLE and CAT combustion sub-systems. The fuel injection and air supply sub-systems are preferably controlled by the control sub-system to selectively inject the fuel and supply air only to the DLE combustion sub-system when the engine is operated under low load conditions and to selectively inject fuel and supply air only to the CAT combustion sub-system when the engine is operated under high load conditions. The fuel injection sub-system is preferably adapted to selectively inject gaseous and liquid fuel to the DLE combustion sub-system and only inject gaseous fuel to the CAT combustion sub-system.
The separately operated CAT combustion sub-system and the DLE combustion sub-system are preferably integrated into one single combustor can. The CAT combustion sub-system is solely used for the power range from switch-over level to full engine power. No pre-burner is required to increase compressor discharge air temperature for the adequate catalyst inlet temperature under engine part power conditions. The specifically designed and optimized combustor scroll cooling and air bypass permit control of the catalyst inlet temperature within the narrow catalyst operating conditions for engine loads between switch-over and full power load. Below the switch-over load the separate DLE combustion sub-system takes over the combustion process control to ensure highest efficiency, lowest NOx emissions, and engine operability, ignition and start up. The present invention combines the advantages of the catalytic and more conventional lean-premixed combustion technologies to produce lowest emission levels over the entire engine operating range from idle to full power, for liquid and gaseous hydrocarbon fuels.
Other advantages and features of the present invention will be better understood with reference to a preferred embodiment described hereinafter.